Injection molding method and injection mold

ABSTRACT

An injection molding method and an injection mold used therefor are provided, in which a molded product free from burrs, whitening and gate marks can be obtained with a simple structure mold, and it can adequately serve the needs for multicavity molding as well. The injection molding method comprises the steps of: introducing and charging a molten resin material into a resin reservoir and a molding cavity of an injection mold, a depth of the resin reservoir being larger than a thickness of a communicating portion; and moving a cut punch, when a portion of the resin material in the resin reservoir is still molten, to push the molten resin in the resin reservoir back from a gate into a runner so that the cut punch not only closes the communicating portion but also cuts a resin solidified portion in the resin reservoir away from a resin molded product in the molding cavity at the communicating portion.

This is a continuation of application Ser. No. 09/622,360 filed Aug. 16,2000. The entire disclosure of the prior application, application Ser.No. 09/622,360 is considered part of the disclosure of the accompanyingcontinuation application and is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to an injection molding method and aninjection mold.

BACKGROUND ART

Injection molding methods have heretofore been used as a molding methodfor producing resin molded products. It is well known that an injectionmolding method is a method involving the steps of: forming a cavitybetween a fixed die and a movable die; charging a molten thermoplasticresin material from a gate provided on part of the cavity by injection;and solidifying the thermoplastic resin material into a predeterminedshape corresponding to a space in the cavity to obtain a resin moldedproduct; and taking the resin molded product out.

Further, injection molds used in injection molding methods come in twotypes; one having means for heating part of a runner that introduces amolten resin material into a cavity from an injection molding apparatus,and the other not having such means. It is also well known that a methodusing the former type of mold having the heating means is called a hotrunner system and that a method using the latter type of mold not havingthe heating means is called a cold runner system.

By the way, immediately after the end of an injection molding processbased on the cold runner system, a resin molded product formed in thecavity and a resin portion solidified in the runner are in one piece.Therefore, the resin molded product and the solidified resin portion inthe runner must be cut into separate pieces at a gate portion, and thussome kind of mechanical cutting means is usually provided in the mold.

On the other hand, the injection molding method based on the hot runnersystem has a mold structure so that the resin in the runner can be keptmolten. Therefore, the cutting means is not generally required. However,to fabricate a molded product having an opening in its part, it ispreferred not to form weld lines (lines formed in the cavity by theconfluence of molten resin portions flowing in different directions, andit is desirable to minimize the formation of such lines from theviewpoints of external appearance and mechanical strength). To preventthe formation of weld lines, it is desired that molding be done using amold in which the gate is previously provided so as to correspond to anopening forming portion and that the portion be thereafter cut away toform the opening. In this case, even the hot runner system must havemechanical cutting means similarly to the cold runner system.

Thus, irrespective of the systems, whether it is the cold runner systemor the hot runner system, various types of methods and molds thatinvolve the cutting operation are proposed in injection molding methods.For example, Japanese Patent Application Laid-open No. Sho 55-15834proposes a molding method for obtaining a ring-like molded product. Inthis molding method, a molten resin is charged into a cavity through asprue bush, and then the sprue bush is retracted as a cut pin forcutting a solidified resin portion near a gate portion moves forward, sothat the gate portion is sheared to separate a subrunner from a resinmolded product. Further, Japanese Patent Application Laid-open No. Hei6-278177 proposes a method that provides a second bush. In this method,a cut bush for cutting a solidified resin portion near a gate portion ismoved forward, and in synchronism with such forward movement of the cutbush, the second bush retracts from a cavity in such a manner as tomechanically absorb an amount of resin pushed out by the cut bush. Stillfurther, Japanese Patent Application Laid-open No. Hei 2-67115 proposesa mold that cuts a number of disk gates. That is, not only a sprue bushis operated while interlocked with the movement of a fixed die away froma fixed table, but also a punch cutter provided on a movable die isprojected, so that a number of disk gates are cut.

Furthermore, Japanese Patent Application Laid-open No. Sho 58-158231proposes a method for separating a molded product from a disk gateformed in a cavity. That is, in a gate portion within the cavity, a gatecutting blade provided on a movable die so as to face the inner surfaceof a fixed die is operated to bring a front end of the blade intocontact with the fixed die, so that the gate disk is cut away.

Furthermore, Japanese Patent Application Laid-open No. Hei 8-294944proposes a method for obtaining a resin molded product having an openingat a central portion. In this method, a specially shaped core pin havingan external shape corresponding to the opening of the molded product iscaused to shuttle between a first position and a second position. Thefirst position stops at a gate a flow of molten resin injected into ahot runner. The second position opens the gate to introduce the moltenresin into a cavity, and is located at the opening portion that is thecenter of axis corresponding to a disk gate portion. Moreover, JapanesePatent Application Laid-open No. Hei 7-276437 proposes a method forobtaining a resin molded product having an opening at the center ofaxis. In this method, a fixed pin is slidably provided in a hot runnerprovided in a fixed die and serves also as a hot gate, and a movable pinis provided on a movable die so as to be slidable in correspondence withthe fixed pin and has a distal end formed so that the distal end engageswith and abuts against a distal end of the fixed pin so as to beconnected to and disconnected from the distal end of the fixed pin. Thedistal end of the fixed pin and that of the movable pin are engaged withand abutted against each other in the cavity so as to correspond to theopening portion of the molded product, so that the resin molded producthaving the opening at the center of axis is obtained.

Still further, Japanese Patent Application Laid-open No. Hei 1-99821proposes a method for cutting a gate portion using a sleeve-like cutterthat is driven toward a fixed die from a movable die after a resinmaterial charged into a cavity has substantially solidified and before aresin portion at the gate portion completely solidifies.

However, the methods and mold disclosed in Japanese Patent ApplicationLaid-opens Nos. Sho 55-15834, Hei 6-278177 and Hei 2-67115 are based onthe concept that complicates the mold structure in order to achieve theabove object, and thus elevates the cost and is highly likely to impairmechanical reliability. Further, if such methods and mold are applied tomulticavity molds, the structure becomes further complicated. Thus, onecan easily guess that the cost increases and the mechanical reliabilityimpairs significantly.

Still further, the method disclosed in Japanese Patent ApplicationLaid-open No. 58-158231 proposes the use of a simply structured mold,but imposes not only a problem that the gate cutting blade comes incontact with the mold when cutting the gate portion, and thus hasdifficulty stably cutting a solidified film layer of the resin material,but also a problem that the fixed die could be damaged by the gatecutting blade abutting against the mold strongly.

The methods disclosed in Japanese Patent Application Laid-open Nos. Hei8-294944 and Hei 7-276437 impose a problem that the molds are expensivesince both methods require an exclusive valve gate system havingspecially shaped or structured pins. The methods also impose, e.g., aproblem that burrs are likely to be produced at a punched portion sincean opening is punched before the gate portion solidifies.

Still further, the method disclosed in Japanese Patent ApplicationLaid-open No. 1-99821 proposes a simply structured mold, but imposes aproblem that the method is effective when there is an amount of resinplenty enough to absorb the driving stroke of the sleeve-like cutter atthe disk gate portion, but if the volume of the disk gate portion isrelatively small, the cutter is hard to drive and thus the method is notapplicable.

DISCLOSURE OF THE INVENTION

Therefore, an object of the present invention is to provide an injectionmolding method that can obtain a molded product free from burrs,whitening and gate marks using a simply structured mold, and that canadequately serve the needs for multicavity molding.

Further, another object of the present invention is to provide aninjection mold beneficially applicable to the aforementioned moldingmethod.

To overcome the aforementioned problems, one aspect of the presentinvention provides an injection molding method wherein a molten resinmaterial, which is injected into a cavity defined by a fixed die and amovable die via a runner provided in the fixed die, is introduced from agate provided in the fixed die and is molded, and the method ischaracterized in that:

when the molten resin material is being charged into the cavity throughthe resin reservoir from the gate, a cut punch, which is provided on theside of the movable die that confronts the gate through a resinreservoir formed by recessing the fixed die toward the gate and which ismovably provided so that the cut punch is inserted into the resinreservoir so as to be in slidable contact with the resin reservoir, hasa distal end thereof extending in a moving direction thereof that istoward the resin reservoir, and the distal end is located between theresin reservoir and the cavity at such a position as to open acommunicating portion that allows the resin reservoir and the cavity tocommunicate with each other so that the molten resin material isintroduced into the cavity via the resin reservoir; and

when the resin material that is still molten is present in the resinreservoir after the molten resin material has been charged into thecavity and the resin reservoir, the cut punch moves toward the gate sothat the cut punch is inserted into the resin reservoir, whereby the cutpunch not only closes the communicating portion while forcibly pushingthe still molten resin material present in the resin reservoir back intothe gate, but also cuts the resin material at the communicating portionso that a resin molded product formed in the cavity is separated from aresin solidified portion formed in the resin reservoir.

Further, another aspect of the present invention provides an injectionmold wherein a fixed die and a movable die form a cavity into which amolten resin material injected via a runner provided in the fixed die isintroduced from a gate provided in the fixed die, and the mold ischaracterized in that:

the gate is connected to the cavity through a resin reservoir formed byrecessing the fixed die toward the gate;

a cut punch is provided on the side of the movable die that confrontsthe gate through the resin reservoir, the cut punch being movable sothat the cut punch can be inserted into the resin reservoir so as to bein slidable contact with the resin reservoir;

when the molten resin material is being charged into the cavity, adistal end of the cut punch extending in a moving direction of the cutpunch that is toward the resin reservoir is located between the resinreservoir and the cavity at such a position as to open a communicatingportion that allows the resin reservoir and the cavity to communicatewith each other so that the molten resin material is introduced into thecavity via the resin reservoir; and when the resin material that isstill molten is present in the resin reservoir after the molten resinmaterial has been charged into the cavity and the resin reservoir, thecut punch moves toward the gate so that the cut punch is inserted intothe resin reservoir, whereby the cut punch not only closes thecommunicating portion while forcibly pushing the still molten resinmaterial present in the resin reservoir back into the gate, but alsocuts the resin material at the communicating portion so that a resinmolded product formed in the cavity is separated from a resin solidifiedportion formed in the resin reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway view of cold runner type injection mold used in thefirst embodiment.

FIG. 2 is a cutaway sectional view of main portion in the cold runnertype injection mold used in the first embodiment.

FIG. 3 is a illustrative view of cavity and resin reservoir in the coldrunner type injection mold used in the first embodiment.

FIG. 4 to 7 respectively show a process of the first embodiment.

FIG. 8 is a illustrative view of hot runner type injection mold used inthe second embodiment.

FIG. 9 is a cutaway sectional view of main portion in the hot runnertype injection mold used in the second embodiment.

FIG. 10 to 13 respectively show a process of the second embodiment.

FIG. 14 a, FIG. 14 b and FIG. 14 c respectively show preferedemobodeiment of a sectional form of the resin reservoir in the injectionmold of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

An injection molding method of the present invention (hereinafterreferred to as the “method of the present invention”) and an injectionmold of the present invention will be described below in details.

In the method of the present invention, a molten resin material injectedby an injection molding machine is charged into a cavity defined by afixed die and a movable die from a gate provided in the fixed die via arunner provided in the fixed die. At this time, the molten resinmaterial introduced from the gate is charged into the cavity whileflowing into a resin reservoir that is formed on the side of the gate inthe fixed die. When the molten resin material is introduced into thecavity via the resin reservoir, a communicating portion that allows themolten resin material to pass through a space between the resinreservoir and the cavity is formed. A cut punch is provided on the sideof the movable die that confronts the gate through the resin reservoirand is movable so that the cut punch is inserted into the resinreservoir so as to be in slidable contact with the resin reservoir inorder to form the communicating portion, and the distal end of such cutpunch is arranged on the side of the movable die. As shown in, e.g.,FIG. 2 to be referred to later, when a molten resin material is chargedinto a cavity 10 via a resin reservoir 12 with a cut punch 15 not yet inoperation, a distal end 23 of the cut punch 15 is held on the side ofthe movable die in such a position as to open a communicating portion 11through which the resin passes from the resin reservoir 12 to the cavity10. Then, as shown in, e.g., FIG. 4 to be referred to later, after themolten resin material has been charged into the cavity and the resinreservoir, the resin material gradually solidifies from a portion thatis in direct contact with the cooled dies toward the inner portion. Atthis time, when the resin material that is still molten is present inthe resin reservoir, the cut punch is operated so that the punch movestoward the gate in such a manner as to be inserted into the resinreservoir. Then, as shown in, e.g., FIG. 5, the cut punch closes thecommunicating portion while forcibly pushing the still molten resinmaterial present in the resin reservoir back into the gate. At the sametime, the cut punch cuts the resin material at the communicatingportion, so that a resin molded product formed in the cavity isseparated from a resin solidified portion formed in the resin reservoir.

In an injection mold used in the method of the present invention, thedepth as viewed in a moving direction of the cut punch in the resinreservoir (i.e., a distance between the distal end of the cut punch thatis not yet in operation and held on the side of the movable die and asurface on the side of the gate of the fixed die forming the resinreservoir), e.g., a distance L₁ shown in FIG. 2 and an opening distanceof the communicating portion in the moving direction of the cut punch,e.g., a distance L₂ shown in FIG. 2 are arranged so that L₁>L₂. As aresult of such arrangement, the solidifying speed of the resin materialin the resin reservoir becomes completely lower than that of the resinmaterial in the communicating portion. Hence, even if the resin materialin the communicating portion has solidified almost completely orhalf-solidified, the resin material that is still molten is present inthe resin reservoir. If the cut punch is operated to move toward thefixed die under such condition, the still molten resin material isforcibly pushed back into the runner via the gate, and this gives thecut punch a moving stroke. Therefore, the larger the thickness of theresin reservoir is when compared with the opening distance of thecommunicating portion, the larger the difference between the solidifyingtime of the resin material in the resin reservoir and that of the resinmaterial in the communicating portion becomes. As a result, sucharrangement is advantageous in increasing the range of timings at whichthe cut punch is stably operated. On the other hand, the resin reservoiris scrapped every time the molding process ends, and thus it is noteconomically desirable to make it thicker than necessary. Thus, as arange in which the molding process can be performed stably andeconomically and in which burring and whitening do not occur at such apart of a resin molded product as to correspond to the communicatingportion, the depth as viewed in the moving direction of the cut punch inthe resin reservoir is preferably set from 1.5 to 10 times the openingdistance of the communicating portion, or more preferably from 2 to 6times.

Further, in the injection mold used in the method of the presentinvention, as shown in FIG. 14 a, such a cross section of the resinreservoir that the depth as viewed in the moving direction of the cutpunch 15 in the resin reservoir 12 is gradually increased toward thegate 14 is advantageous in increasing the operating area of the cutpunch. Further, as shown in FIG. 14 b, such a cross section that thedepth of the outermost peripheral portions 12 a at both ends of theresin reservoir 12 is increased is also advantageous in ensuring theoperating stroke of the cut punch. Still further, as shown in FIG. 14 c,such a cross section that the depth as viewed in the moving direction ofthe cut punch 15 in the resin reservoir 12 is gradually increased towardthe gate 14 and the depth of the outermost peripheral portions 12 a atboth ends of the resin reservoir 12 is increased may also be acceptableand is advantageous in ensuring the operating stroke of the cut punch.

Still further, it is so arranged that the diameter of the shaft of apusher pin for pushing the resin solidified portion in the resinreservoir is increased so that the area of projection of the distal endof the moving cut punch as viewed in the moving direction of the cutpunch is decreased, and that the cut punch is operated independently ofthe pusher pin so that when the cut punch operates, the pusher pin doesnot operate. Such an arrangement is advantageous in ensuring theoperating area and stroke of the cut punch.

In the method of the present invention, when the resin material in thecommunicating portion between the resin reservoir and the cavitysemi-solidifies and the resin material that is still molten is presentin the resin reservoir after the charging of the molten resin materialinto the cavity has been completed, the cut punch is operated to movetoward the gate of the fixed die, so that the cut punch closes thecommunicating portion. The operating timing of the cut punch is selectedappropriately in accordance with the shape of the molding cavity, i.e.,the shape of a resin molded product, the solidifying speed of the resinmaterial, the shape of the resin reservoir, and the like. Generally, ifthe cut punch is operated when the resin material in the communicatingportion is still molten, i.e., if the cut punch is operated at an earlytiming, the resin material creeps into an operating clearance providedbetween a side surface of the resin reservoir and a side surface of thecut punch since the mold is designed to have a slight clearance frommanufacturing and structural considerations. As a result, there is adanger that burrs are formed at the cut end of the resin molded product.

Further, if the clearance is decreased to overcome the burring problem,the side surface of the resin reservoir abuts against the cut punch tocause scorings, and in the extreme case, there may be an operationproblem, e.g., that the communicating portion of the fixed die isdamaged. In addition, if the cut punch is operated after the resinmaterial in the communicating portion has completely solidified, i.e.,if the cut punch is operated at a later timing, tiny cracks are presentat the cut end of a resin molded product obtained and whitening appearsover the exterior of the product. Therefore, it is important todetermine while confirming the operating timing of the cut punch for themethod of the present invention through tests and the like so that thetiming comes later than when the cut end burrs and earlier than when thecut end whitens in the resin molded products obtained.

In the method of the present invention, the larger the diameter of theopening of the gate through which the molten resin material passes whenthe resin material that is still molten in the resin reservoir is pushedback into the runner by the operation of the cut punch, the easier themolten resin material is caused to flow back into the runner, and thusthe larger diameter of the opening of the gate is advantageous inoperating the cut punch as desired. On the other hand, in the case wherea cold runner type injection mold is used, a larger diameter of theopening of the gate is likely to make it hard to separate the resinsolidified portion formed in the resin reservoir from the runner. Toovercome this problem, in the case where a cold runner type injectionmold is used, it is preferred to previously fabricate the mold having anordinary pin gate diameter, examine the operating conditions of the cutpunch by checking the burring or whitening conditions of an obtainedmolded product, and determine an appropriate opening diameter byincreasing the diameter as necessary. This applies similarly to thediameter of the opening of the gate of the injection mold of the hotrunner structure. In the case where a hot runner type injection moldhaving a valve gate is used, there is no need to consider the problem ofseparating the resin solidified portion in the resin reservoir from therunner at the gate since the gate is mechanically closed by a needlepin. Therefore, it is preferred to increase the diameter of the openingof the gate as much as possible. For example, the diameter of theopening of the gate preferably ranges from 1.0 to 2.5 mm.

Embodiments of the Invention

An injection molding method and an injection mold, which are preferredembodiments 1 and 2 of the present invention will now be described basedon FIGS. 1 to 13. FIGS. 1 to 7 show a first embodiment for fabricating abent plate-like resin molded product using a cold runner type mold thatis a preferred embodiment of a mold of the present invention. FIGS. 8 to13 show a second embodiment for fabricating a thin annular part using ahot runner type mold.

Cold runner type injection mold 1 used in the first embodiment whosemain portion is shown in FIG. 1 in the form of a cutaway sectionincludes a fixed die 2 and a movable die 3. The fixed die 2 includes afixed mounting plate 2 a and a fixed die plate 2 c mounted on the fixedmounting plate 2 a through a fixed back plate 2 b. A sprue bush 6 inwhich an introducing hole 5 is formed is provided on the fixed mountingplate 2 a. The introducing hole 5 introduces a molten resin materialinjected from an injection nozzle 4 of an injection molding machine intothe mold. The introducing hole 5 of the sprue bush 6 is formed insidethe fixed back plate 2 b and the fixed die plate 2 c, and communicateswith a runner 7 through which the molten resin material flows.

The movable die 3 has a movable mounting plate 3 a and a movable dieplate 3 c that is mounted on the movable mounting plate 3 a through aspacer block 3 b.

A resin reservoir 12 is formed by recessing the fixed die plate 2 c ofthe fixed die. That is, at the time the fixed die 2 and the movable die3 of the injection mold 1 are matched, a stepped molding surface 8formed on the fixed die plate 2 c of the fixed die 2 and a steppedmolding surface 9 formed on the movable die plate 3 c of the movable die3 so as to correspond to the molding surface 8 forms the resin reservoir12 that communicates to a cavity 10 that forms the resin molded productsthrough the communicating portion 11 as shown by a plan view of FIG. 3.In FIG. 3, reference numeral 14 denotes a gate for introducing themolten resin material into the cavity.

Further, as shown in FIG. 2, the resin reservoir 12 is connected to therunner 7 via the gate 14 formed in a resin reservoir surface 13 on themolding surface 8 of the fixed die plate 2 c of the fixed die. Therunner 7 introduces the molten resin material from the nozzle 4 of theinjection molding machine.

Further, a cut punch 15 is provided on the side of the movable die plate3 c of the movable die 3 that confronts the gate 14 through the resinreservoir 12. The cut punch 15 is so arranged that when the molten resinmaterial is being charged into the cavity, a distal end 23 of the cutpunch is located between the resin reservoir 12 and the cavity 10 atsuch a position as to open a communicating portion 11 that allows theresin reservoir 12 and the cavity 10 to communicate with each other sothat the molten resin material is introduced into the cavity via theresin reservoir 12, and that when the resin material that is stillmolten is present in the resin reservoir 12 after the molten resinmaterial has been charged into the cavity, the cut punch is moved fromthe movable die 3 to the fixed die 2 so that the resin reservoir 12 isseparated from the cavity 10 at the communicating portion 11. The cutpunch 15 is secured to a cut punch driving hydraulic cylinder 17together with cut punch pusher plates 16 a and 16 b, and is urged by aspring 18 in a direction opposite to a driving direction.

Further, a resin reservoir pusher pin 19 for pushing out a resinsolidified portion formed in the resin reservoir 12 is inserted into thecut punch 15 so as to be slidable independently of the cut punch 15.Also, molded product pusher pins 20 a and 20 b for pushing out a resinmolded product formed in the cavity 10 are slidably inserted into themovable die plate 3 c. The resin reservoir pusher pin 19 and the moldedproduct pusher pins 20 a and 20 b pass through the cut punch pusherplates 16 a and 16 b and are mounted on pusher plates 22 a and 22 b thatare urged by a spring 21 in a direction opposite to a driving directionof the pusher pins. The resin reservoir pusher pin 19 and the moldedproduct pusher pins 20 a and 20 b are driven forward (in the elevatingdirection as viewed in FIG. 1) when a pushing rod (not shown) of themolding machine pushes first a pusher rod receiver 22 c and then thepusher plate 22 b that is integrally mounted on the pusher rod receiver22 c.

As shown in FIGS. 1 and 2, undercut portions 25 a and 25 b are formed inthe distal end 23 of the cut punch 15. The portions 25 a and 25 b allowthe cut punch to hold the resin solidified portion formed in the resinreservoir 12 so that the resin solidified portion does not remain on theside of the fixed die at the time of mold opening.

In the injection mold 1, the communicating portion 11 provided betweenthe resin reservoir 12 and the cavity is arranged so that the depth asviewed in the moving direction of the cut punch 15 in the resinreservoir becomes larger than the opening distance of the communicatingportion in the moving direction of the cut punch as shown in FIG. 2.That is, a distance L₁ between a molding surface 8 a of the fixed dieplate 2 c on a side surface 26 of the resin reservoir 12 and the distalend 23 of the cut punch 15 is arranged to be larger than the openingdistance of the communicating portion 11 extending in the movingdirection of the cut punch, i.e., a distance L₂ between a moldingsurface 8 b of the fixed die plate 2 c and the distal end 23 of the cutpunch 15 (L₁>L₂). As a result of such arrangement, when the cut punch 15is driven in a direction indicated by an arrow A, not only a sidesurface 15 a of the cut punch 15 closes the communicating portion 11,but also cuts the resin molded product formed in the cavity 10 away fromthe resin solidified portion formed in the resin reservoir 12 at thecommunicating portion 11.

In the injection molding method using the injection mold 1 that has thecold runner structure shown in FIG. 1, first, the movable die 3 isdriven to close the fixed die 2 and the movable die 3, which in turnforms the cavity 10 and the resin reservoir 12 communicating with thecavity 10. Then, the molten resin material is injected from theinjection nozzle 4 of the injection molding machine, the nozzle 4 beingbrought into contact with the introducing hole 5 that is formed in thesprue bush 6 provided in the fixed mounting plate 2 a. The injectedmolten resin material is introduced into the resin reservoir 12 from thegate 14 while flowing through the runner 7, and is further charged intothe cavity 10 via the communicating portion 11. At this time, the cutpunch 15 does not operate and is held on the side of the movable die 3.

The resin material charged into the resin reservoir 12, thecommunicating portion 11 and the cavity 10 is cooled by cooling watercirculating through cooling means, e.g., cooling water passagesappropriately provided within the fixed die 2 and the movable die 3, andthus solidified. After the charging of the resin material has beencompleted, the charged resin material is cooled to solidify, as shown inFIG. 4, from the outside portion that is in contact with the moldingsurface 8 of the fixed die plate 2 c and the molding surface 9 of themovable die plate 3 c, and the resin material at the communicatingportion 11 where the molding surface 8 neighbors the molding surface 9solidifies or semi-solidifies first. On the other hand, in the resinreservoir 12 that is thicker than the communicating portion 11, i.e., inthe resin reservoir 12 in which the distance between the molding surface8 of the fixed die plate 2 c and the molding surface 9 of the movabledie plate 3 c is large, the outside portion of the resin material thatis in contact with the molding surfaces 8 and 9 solidifies orsemi-solidifies, but a resin material portion 27 that is still molten ispresent in the inside.

At this time, the cut punch 15 that is mounted on the cut punch pusherplates 16 a and 16 b is operated by the cut punch driving hydrauliccylinder 17 against the urging force of the spring 18, so that the cutpunch moves from the movable die 3 to the fixed die 2. The still moltenresin material 27 in the resin reservoir is pushed back from the gate 14to the runner 7 by the pushing force of the moving cut punch 15. As aresult, the volume of the resin in the resin reservoir is reduced, whichin turn provides a stroke for allowing the cut punch 15 to move. Hence,as shown in FIG. 5, the cut punch 15 advances toward the fixed die plate2 c, so that the side surface 15 a of the cut punch 15 closes thecommunicating portion 11 and a resin molded product 28 in the cavity 10is cut away from a resin solidified portion 29 formed in the resinreservoir 12 at the communicating portion 11. At this time, the cutpunch 15 is mounted with its base 30 held between the cut punch pusherplates 16 a and 16 b shown in FIG. 1, so that the moving distance of thecut punch 15 is regulated by the distance between an outside surface 31of the cut punch pusher plate 16 a and a rear surface 32 of the movabledie plate 3 c. The resin reservoir pusher pin 19 inserted into the cutpunch 15 so as to be slidable independently of the cut punch 15 does notmove together with the cut punch 15 when the cut punch moves, and thus atop face 24 of the pin 19 forms a recess 33.

The resin material in the cavity 10 and the resin reservoir 12 is cooledand solidifies, so that the resin molded product 28 is formed in thecavity 10, and the resin solidified portion 29 including a resinmaterial portion solidified at the recess 33 and the undercut portions25 a and 25 b is formed in the resin reservoir 12. Then, as shown inFIG. 6, the movable die 3 is driven to open the injection mold 1. Atthis time, the resin solidified portion 29 formed in the resin reservoiris separated from the resin molded product, and held on the distal endof the cut punch 15. Further, the solidified portion formed in theundercut portions 25 a and 25 b functions as a holding portion forholding the resin solidified portion 29 on the distal end of the cutpunch 15.

Next, as shown in FIG. 7, the pusher rod (not shown) of the moldingmachine drives the resin reservoir pusher pin 19 and the molded productpusher pins 20 a and 20 b mounted on the pusher plates 22 a and 22 bagainst the urging force of the spring 21, and the resin molded product28 and the resin solidified portion 29 are pushed out independently ofeach other and obtained in the form of cut pieces.

Further, FIGS. 8 to 13 show the second embodiment for fabricating thinannular parts using a hot runner type four-cavity mold based on a valvegate system. FIG. 8 shows only a half of the four-cavity injection mold,omitting the other half on the back of the sheet. The followingdescription is based on the half of the mold with two cavities shown inFIG. 8, and a description of the other half on the back of the sheet isomitted.

A hot runner type injection mold 41 used in the second embodiment whosemain portion is shown in FIG. 8 in a cutaway form includes a fixed die42 and a movable die 43. The fixed die 42 has a fixed mounting plate 42a and a fixed die plate 42 c mounted on the fixed mounting plate 42 athrough a spacer block 42 b. A sprue bush 46 in which an introducinghole 45 is formed is provided on the fixed mounting plate 42 a. Theintroducing hole 45 introduces a molten resin material injected from aninjection nozzle 44 of an injection molding machine into the mold. Theintroducing hole 45 of the sprue bush 46 is connected to a runner 48that is provided in a manifold block 47 interposed between the fixedmounting plate 42 a and the fixed die plate 42 c, and communicates withhot runners 50 a and 50 b of hot runner nozzles 49 a and 49 b providedin the fixed die plate 42 c. Needle pins 53 a and 53 b are inserted intothe hot runners 50 a and 50 b of the hot runner nozzles 49 a and 49 b,respectively. The pins 53 a and 53 b are driven by needle pin drivingcylinders 51 a and 51 b provided in the fixed mounting plate 42 a andcontrol the introduction of the molten resin material into resinreservoirs 58 a and 58 b, and cavities 56 a and 56 b which will bedescribed later.

The movable die 43 includes a movable mounting plate 43 a and a movabledie plate 43 e secured to movable back plates 43 c and 43 d that aremounted on the movable mounting plate 43 a through a spacer block 43 b.

The resin reservoirs 58 a and 58 b are formed by molding surfaces 54 aand 54 b and molding surfaces 55 a and 55 b, respectively. That is, asshown in FIGS. 8 and 9, when the fixed die 42 and the movable die 43 ofthe injection mold 41 are closed, the stepped molding surfaces 54 a and54 b formed on the fixed die plate 42 c of the fixed die 42 and thestepped molding surfaces 55 a and 55 b formed on the movable die plate43 e of the movable die 43 so as to correspond to the molding surfaces54 a and 54 b form the resin reservoirs 58 a and 58 b that communicatewith annular cavities 56 a and 56 b for forming resin molded productsthrough communicating portions 57 a and 57 b. Further, gates 52 a and 52b are formed in resin reservoir surfaces 59 a and 59 b of the moldingsurfaces 54 a and 54 b of the resin reservoirs 58 a and 58 b on the sideof the fixed die plate 42 c, respectively.

The resin reservoirs 58 a and 58 b are connected to the hot runners 50 aand 50 b via the gates 52 a and 52 b formed in the resin reservoirsurfaces 59 a and 59 b of the molding surfaces 54 a and 54 b on thefixed die plate 42 c of the fixed die 42. The hot runners introduce themolten resin material.

Cut punches 60 a and 60 b are provided on the side of the movable dieplate 43 e of the movable die 43 that confronts the gates 52 a and 52 bthrough the resin reservoirs 58 a and 58 b. The cut punches 60 a and 60b are so arranged that when the molten resin material is being chargedinto the cavities, distal ends 67 a and 67 b of the punches are locatedbetween the resin reservoirs and cavities at such positions as to openthe communicating portions that allow the resin reservoirs 58 a and 58 band the cavities 56 a and 56 b to communicate with each other so thatthe molten resin material is introduced into the cavities via the resinreservoirs 58 a and 58 b, and that when the resin material that is stillmolten is present in the resin reservoirs 58 a and 58 b after the moltenresin material has been charged into the cavities, the cut punches aremoved from the movable die 43 to the fixed die 42 so that the resinreservoirs 58 a and 58 b are separated from the cavities 56 a and 56 bat the communicating portions 57 a and 57 b. The cut punches 60 a and 60b are connected to cut punch driving hydraulic cylinders 61 a and 61 bprovided in the movable back plate 43 c together with the movable backplates 43 c and 43 d, and are urged by springs 62 a and 62 b in adirection opposite to a driving direction.

Further, resin reservoir pusher pins 63 a and 63 b for pushing out resinsolidified portions formed in the resin reservoirs 58 a and 58 b areinserted into the cut punches 60 a and 60 b so as to be slidableindependently of the cut punches 60 a and 60 b. Also, molded productpusher sleeves 64 a and 64 b for pushing out resin molded productsformed in the cavities 56 a and 56 b are slidably inserted into themovable die plate 43 e.

The resin reservoir pusher pins 63 a and 63 b and the molded productpusher sleeves 64 a and 64 b pass through the movable back plates 43 cand 43 d and are mounted on pusher plates 66 a and 66 b urged by aspring 65 in a direction opposite to a moving direction of the pusherpins and sleeves.

The resin reservoir pusher pins 63 a and 63 b and the molded productpusher sleeves 64 a and 64 b are driven forward (in the elevatingdirection as viewed in FIG. 8) when a pushing rod (not shown) of themolding machine pushes first a pusher rod receiver 66 c and then thepusher plates 66 a and 66 b that are integrally mounted on the pusherrod receiver 66 c.

As shown in FIGS. 8 and 9, undercut portions 69 a and 69 b are formed inthe distal ends 67 a and 67 b of the cut punches 60 a and 60 b. Theportions 69 a and 69 b allow the cut punches 60 a and 60 b to hold theresin solidified portions formed in the resin reservoirs 58 a and 58 bso that the resin solidified portions do not remain on the side of thefixed die at the time of mold opening.

Further, in the injection mold 41, the communicating portion 57 aprovided between the resin reservoir 58 a and the cavity 56 a and thecommunicating portion 57 b provided between the resin reservoir 58 b andthe cavity 56 b are arranged so that the depth of the resin reservoirsbecome larger than the opening distance of the communicating portions inthe moving direction of the cut punches 60 a and 60 b. That is, when thecut punches 60 a and 60 b are not operated and held on the side of themovable die plate 43 e, a distance L₃ between the molding surface 54 aor 54 b of the fixed die plate 42 c on a side surface 101 of the resinreservoir 58 a or 58 b and the distal end 67 a or 67 b of the cut punch60 a or 60 b is arranged to be larger than the opening distance of thecommunicating portion 57 a or 57 b extending in the moving direction ofthe cut punch, i.e., a distance L₄ between the molding surface 54 a or54 b of the fixed die plate 43 e and the distal end 67 a or 67 b of thecut punch 60 a or 60 b (L₃>L₄). As a result of such arrangement, the cutpunch 60 a is operated to move in a direction indicated by an arrow A,so that a side surface 102 a of the cut punch 60 a not only closes thecommunicating portion 57 a, but also cuts the resin molded productformed in the cavity 56 a away from the resin solidified portion formedin the resin reservoir 58 a at the communicating portion 57 a. Further,a similar operation is performed at the resin reservoir 58 b and thecavity 56 b.

In the injection molding method using the injection mold 41 that has thehot runner structure shown in FIG. 8, first, the movable die 43 isdriven to close the fixed die 42 and the movable die 43, which in turnforms the cavities 56 a and 56 b and the resin reservoirs 58 a and 58 bthat communicate with the cavities 56 a and 56 b through thecommunicating portions 57 a and 57 b. Then, the injection nozzle 44 ofthe injection molding machine is brought into contact with theintroducing hole 45 formed in the sprue bush 46 provided in the fixedmounting plate 42 a, and the molten resin material is injected. Theinjected molten resin material is introduced into the hot runners 50 aand 50 b while flowing through the runner 48. At this time, the needlepins 53 a and 53 b are driven upward as viewed in the drawing by theneedle driving cylinders 51 a and 51 b to open the gates 52 a and 52 b,so that the molten resin material is introduced and charged into thecavities 56 a and 56 b via the resin reservoirs 58 a and 58 b and thecommunicating portions 57 a and 57 b. At this time, the cut punches 60 aand 60 b are not operated and held on the side of the movable die 43.

The resin material charged into the resin reservoirs 58 a and 58 b, thecommunicating portions 57 a and 57 b and the cavities 56 a and 56 b iscooled by cooling water flowing through cooling means, e.g., coolingwater passages appropriately provided within the fixed die 42 and themovable die 43, and thus solidified.

Next, an operation at the resin reservoir 58 a, the communicatingportion 57 a and the cavity 56 a will be described as an example. Anoperation at the resin reservoir 58 b, the communicating portion 57 band the cavity 56 b is also similar.

After the charging of the molten resin material has been completed, thecharged resin material is cooled and solidifies, as shown in FIG. 10,from the outside portion that is in contact with the molding surface 54a of the fixed die plate 42 c and the molding surface 55 a of themovable die plate 43 e, and the resin material at the communicatingportion 57 a where the molding surface 54 a neighbors the moldingsurface 55 a solidifies or semi-solidifies first. On the other hand, inthe resin reservoir 58 a that is deeper than the communicating portion57 a, i.e., in the resin reservoir in which the distance between themolding surface 54 a of the fixed die plate 42 c and the molding surface55 a of the movable die plate 43 e is larger than the communicatingportion 57 a, the outside portion of the resin material that is incontact with the molding surfaces 54 a and 55 a solidifies orsemi-solidifies, but a resin material portion 103 a that is still moltenis present in the inside.

At this time, the cut punch 60 a mounted on the movable back plates 43 cand 43 d is moved from the movable die 43 to the fixed die 42 by the cutpunch driving hydraulic cylinder 61 a against the urging force of thespring 62 a. The still molten resin material portion 103 a in the resinreservoir is pushed back into the hot runner 50 a from the gate 52 a bythe pushing force of the driven cut punch 60 a. As a result, the volumeof the resin in the resin reservoir is reduced, which in turn provides astroke for allowing the cut punch 60 a to move. Hence, as shown in FIG.11, the cut punch 60 a moves toward the fixed die plate 42 c, so thatthe side surface 102 a of the cut punch 60 a not only closes thecommunicating portion 57 a, but also cuts a resin molded product 105 ain the cavity 56 a away from a resin solidified portion 106 a formed inthe resin reservoir 58 a at the communicating portion 57 a. At thistime, the resin reservoir pusher pin 63 a that is inserted into the cutpunch 60 a so as to be slidable independently of the cut punch 60 a isnot driven together with the cut punch 60 a when the cut punch 60 a isdriven, and thus a top face 68 a of the pin 63 a forms a recess 107 a.

Then, the needle pin 53 a is driven to close the gate 52 a, and theresin material in the cavity 56 a and the resin reservoir 58 a is cooledand solidifies. Then, the resin molded product 105 a is formed in thecavity 56 a, and the resin solidified portion 106 a including a resinmaterial portion solidified at the recess 107 a and the undercutportions 69 a and 70 a is formed in the resin reservoir 58 a. As shownin FIG. 12, the movable die 43 is thereafter driven to open theinjection mold. At this time, the resin solidified portion 106 a formedin the resin reservoir is separated from the resin molded product 105 a,and held on the distal end of the cut punch 60 a. Further, thesolidified portion formed in the undercut portions 69 a and 70 afunctions as a holding portion for holding the resin solidified portion106 a on the distal end of the cut punch 60 a.

Then, as shown in FIG. 13, the pusher rod (not shown) of the moldingmachine drives the resin reservoir pusher pin 63 a and the moldedproduct pusher sleeve 64 a mounted on the pusher plates 66 a and 66 bagainst the urging force of the spring 62 a, so that the thin annularresin molded product 105 a and the disk-like resin solidified portion106 a are pushed out independently of each other in cut pieces.

EXAMPLES Test Example 1

Tests were carried out to fabricate bent plate-like resin molded productusing the cold runner type injection mold of the present invention shownin FIG. 1 by changing the used resin, mold temperature, clearance of thesliding portion between the inner surface of the resin reservoir and theouter periphery of the cut punch.

The thicknesses of the product forming portion, the communicatingportion and the resin reservoir (at the time of injection) of theinjection mold used for the tests were as follows. Thickness of productforming portion (cavity 10) 1.5 mm Thickness of communicatingportion(11) 0.5 mm Thickness of resin reservoir (12) at the time of 1.0mm injection

In the tests, the cutting quality given by the cut punch and associatedproblems are evaluated based on the following criteria. The results areshown in Table 1.

Cutting quality evaluation criteria:

-   ∘ Satisfactory without cracks and burrs at cut end-   Δ Acceptable in terms of external appearance although with some    cracks and burrs at cut end-   X Defective with cracks and burrs at cut end

Problems:

K Unsatisfactory in terms of mold durability because of scorings atsliding portion

B Burring at cut end of molded product

H Whitening at cut end of molded product or cutting of molded productimpossible TABLE 1 Mold Cutting Used resin temperature Clearance qualityProblems GPPS 50° C. Less than 5 μm ◯ K 5 to 20 μm ◯ None more than 20μm Δ To X B PC 90° C. Less than 5 μm ◯ K 5 to 20 μm ◯ None more than 20μm ◯ None PA 70° C. Less than 5 μm ◯ K 5 to 20 μm ◯ None more than 20 μmΔ To X BNotes:GPPS: General-purpose polystyrenePC: PolycarbonatesPA: Polyamides

It can be said from the results shown in Table 1 that narrowerclearances of the sliding portion between the inner surface of the resinreservoir and the outer periphery of the cut punch provide a bettercutting quality, but too narrow a clearance makes it easy to bring thecut punch into contact with the resin reservoir, and this is notdesirable in terms of mold durability. Particularly, if the clearance ofthe sliding portion is less than 5 μm, a mold of high accuracy must beused to avoid the mold durability problem, and this increases the costfor the mold and hence is not desirable economically. Therefore, wheneconomic advantages are considered, it is desirable to optimize theclearance in accordance with the resin properties used. It has beenverified from the results shown in Table 1 that clearances of 10 to 20μm are preferred for GPPS and PC, whereas clearances of 5 to 10 μm arepreferred for PA.

Further, tests were carried out to fabricate bent plate-like resinmolded products using the same mold by changing the used resin, moldtemperature and cut punch drive timing to evaluate the cutting qualitygiven by the cut punch and associated problems under the same criteria.The results are summarized in Table 2. “Drive timing” means a timeinterval (in seconds) from injection completion (pressure holding start)to cut punch drive start. TABLE 2 Driving Used Mold Timing Cutting resintemperature Clearance (sec) quality Problems GPPS 50° C. 10 μm Less than0.5 X B 0.5 to 1.5 ◯ None more than 1.5 X H PC 90° C. 10 μm Less than0.8 X B 0.8 to 2.5 ◯ None more than 2.5 X H PA 70° C. 10 μm Less than1.5 X B 1.5 to 2.5 ◯ None more than 2.5 X H

It can be said from the results shown in Table 2 that the cut punchdrive timing differs depending on the resin properties. What is commonto the above three kinds of resins is that an early cut punch drivetiming produces burrs at the cut ends and a late cut punch drive timingproduces whitening and a further late timing makes it impossible to cutthe products.

Test Example 2

Tests were carried out to fabricate thin annular parts by using the hotrunner type injection mold of the present invention shown in FIG. 8 andusing PA as a resin at a mold temperature of 70° C. and by changing theclearance of the sliding portion between the inner surface of the resinreservoir and the outer periphery of the cut punch to obtain cut punchdriving timings that can give satisfactory cutting quality. The resultsare shown in Table 3.

The thicknesses of the product forming portion, the communicatingportion and the resin reservoir (at the time of injection) of theinjection mold used for the tests were as follows. Thickness of productforming portion (cavity 56a) 0.17 mm Thickness of communicating portion(57a) 0.17 mm Thickness of resin reservoir (58a) at the time of  0.5 mminjection

TABLE 3 Drive timing Used resin Mold temperature Clearance (sec) PA 70°C.  5 μm 0.2 to 1.4 10 μm 0.4 to 1.5 15 μm 1.5 to 1.6 more than 20 μm Norange

It can be said from the results shown in table 3 that narrowerclearances of the sliding portion between the inner surface of the resinreservoir and the outer periphery of the cut punch provide a wider drivetiming range for giving satisfactory cutting quality, whereas widerclearances provide a narrower drive timing range for giving satisfactorycutting quality. Further, too narrow a clearance makes it easy to bringthe cut punch into contact with the resin reservoir similarly to TestExample 1, which is not desirable in terms of mold durability. When PAis used as a resin, a clearance of 15 μm makes the cuttingquality-optimizing drive timing as short as 0.1 sec, and this is notdesirable in terms of stable production. Further, a clearance of 20 μmprovides no drive timing range that can optimize the cutting quality.Therefore, the optimal cut punch drive timing that can give satisfactoryclearance and cutting quality differs depending on the resin properties.

Further, tests were carried out to fabricate thin annular parts by usingthe same mold and using PA as a resin at mold temperatures of 60 and 80°C. by changing the thickness of the resin reservoir to obtain cut punchdrive timings for giving satisfactory cutting quality. The results areshown in Tables 4 and 5. TABLE 4 Resin reservoir Driving Mold thicknesstiming Used resin temperature Clearance (mm) (sec) PA 60° C. 10 μm 0.20.1 to 0.2 0.25 0.1 to 0.3 0.5 0.3 to 1.2 1.0 0.5 to 2.0 1.7 0.5 to 2.5

TABLE 5 Thickness Resin ratio resin reservoir Drive reservoir to UsedMold thickness timing communicating resin temperature Clearance (mm)(sec) portion PA 80° C. 10 μm 0.2 0.1 to 0.2 1.2 0.25 0.1 to 0.4 1.50.35 0.3 to 1.0 2.0 0.5 0.5 to 1.7 2.5 1.0 0.6 to 2.5 5.9 1.7 0.6 to 2.910

It is understood from the results shown in Tables 4 and 5 that thickerresin reservoirs provide a wider range of cut punch drive timings forgiving satisfactory cutting quality. Further, thickness ratios betweenthe resin reservoir and the communicating portion of 1.5 or less cannotprovide an adequately wide drive timing range, which imposes problems interms of stable production. Still further, the resin in the resinreservoir is scrapped every molding cycle, and thus is a waste.Therefore, there is no need to have a drive timing range wider thannecessary, and thus from the viewpoint of cut punch drivingcontrollability and effective use of resources, it is understood thatthe thickness of the resin reservoir should preferably range from 1.5 to10 times the thickness of the communicating portion, or more preferablyfrom about 2 to 6 times.

INDUSTRIAL APPLICABILITY

As described in the foregoing, the injection molding method of thepresent invention can provide the following advantages.

(1) When the resin is injected and charged, the resin material in theresin reservoir solidifies from its outside surface, and at the time theresin material in the communicating portion that communicates with thecavity has solidified, the molten resin material remains in the resinreservoir since the resin reservoir is deeper than the communicatingportion. When the cut punch is driven under such condition, the moltenresin material in the resin reservoir can flow backward into the runnervia the gate. As a result, the volume of the resin in the resinreservoir can be reduced, so that the moving stroke of the cut punch canbe provided. Hence, the method of the present invention can eliminatethe use of a complicated structure of the conventional mold, e.g., amoving sprue bush employed in a mold for fabricating optical disksubstrates. That is, the method of the present invention can simplifythe mold structure.

(2) If the resin reservoir is considered as part of a product, the gatein the mold of the present invention functions as a gate in an ordinarymold. Therefore, by adding only a cut punch drive system, the mold ofthe present invention can be designed to have a structure as simple asthat of the conventional mold. As a result, the design of a multicavitymold can be similar to that of the conventional example, and thusbecomes easy. Still further, even if a hot runner type is employed, thestructure of the mold of such type can be designed as simple as that ofa conventional example. Therefore, a commercially available hot runnersystem can be directly used, and thus an inexpensive mold can beprovided.

(3) The solidified (or semi-solidified) resin material in thecommunicating portion is cut away. Therefore, the cut end has no burrsinto which the molten resin can creep, and thus a resin molded producthaving no gate marks can be obtained. Further, since the molten resindoes not creep into the cut end, an adequate clearance between the cutpunch and the resin reservoir into which the cut punch is inserted canbe provided, and thus the user does not have to worry about scorings atthis portion. Hence, a durable mold can be provided and mold parts canbe fabricated inexpensively.

1. An injection mold comprising: a fixed die and a movable die whichform a cavity into which a molten resin material is injected to form aresin molded product; a runner through which the molten resin materialsupplied to said fixed die flows, said runner being provided in saidfixed die and passing through inside of said fixed die; a resinreservoir formed by recessing said fixed die toward a opposite side ofsaid movable die and communicating with said runner and said cavity; acommunicating portion formed on a boundary between said resin reservoirand said cavity and allowing said resin reservoir and said cavity tocommunicate with each other so that the molten resin material isintroduced into said cavity via said resin reservoir; a gate formed on aboundary between said runner and said resin reservoir and at an endportion of said runner in said fixed die and injecting to said resinreservoir the molten resin material supplied to said fixed die andflowed through said runner in said fixed die to charge the molten resinmaterial into the cavity through said communicating portion from saidresin reservoir; a cut punch provided on a side of said movable die thatconfronts said gate through said resin reservoir and having a distalend, said cut punch being movable between a first position to close saidcommunicating portion, in which said distal end of said cut punch isinserted into said resin reservoir so as to be in slidable contact withsaid resin reservoir and a second position to open said communicatingportion, in which said distal end of said cut punch is drawn out of andapart from said resin reservoir; an undercut portion provided at aperiphery of said distal end of said cut punch, disposed closer to anedge portion of said distal end than a center portion of said distalend, said undercut portion serving to hold a resin solidified portionformed in said resin reservoir at the time of mold opening; and meansfor driving said cut punch between said first position and said secondposition in a moving direction of said cut punch toward the resinreservoir, wherein said driving means drives said cut punch to locatesaid distal end at said second position between said resin reservoir andsaid cavity to open said communicating portion in such a way that whenthe molten resin material is being charged into said cavity, said resinreservoir and said cavity are allowed to communicate with each other sothat the molten resin material is introduced into said cavity via saidresin reservoir, wherein said driving means drives said cut punch tomove said distal end of said cut punch toward said first position afterthe molten resin material has been charged into said cavity and saidresin reservoir, and to locate said distal end at said first position insaid resin reservoir to close said communicating portion in such a waythat when an inner portion of the resin material that is present in saidresin reservoir is still molten, a portion of the resin material thereofthat is in direct contact with said cooled dies is gradually solidifiedand thereby the portion of the resin material at said communicatingportion between said cavity and said resin reservoir is semi-solidified,said cut punch is inserted into said resin reservoir, whereby said cutpunch not only closes said communicating portion while forcibly pushingthe still molten resin material present in said resin reservoir backthrough said gate into said runner, but also cuts the resin material atsaid communicating portion so that said resin molded product formed insaid cavity is separated from said resin solidified portion formed insaid resin reservoir, and wherein said undercut portion of said distalend of said cut punch holds the resin solidified portion formed in saidresin reservoir.
 2. The injection mold according to claim 1, furthercomprising: a sprue bush provided on said fixed die and having anintroducing hole that introduces the molten resin material supplied tosaid fixed die, wherein said runner communicates with said introducinghole of said sprue bush and the molten resin material supplied to saidfixed die and introduced into said introducing hole of said sprue bushflows through said runner in said fixed die.
 3. The injection moldaccording to claim 2, wherein said sprue bush introduces into saidintroducing hole thereof the molten resin material supplied to saidfixed die from an injection nozzle of an injection molding machine. 4.The injection mold according to claim 1, comprising plural pairs of thecavities, the resin reservoirs and the cut punches.
 5. The injectionmold according to claim 1, wherein said runner is a cold runner and hasa tapered portion on an end side of said runner inside said fixed die,and said gate is formed at said end portion of said tapered portion ofsaid runner.
 6. The injection mold according to claim 5, wherein saidgate is closed with a semi-solidified resin material or a solidifiedresin material after said cut punch has moved to said first position. 7.The injection mold according to claim 1, wherein said runner is a hotrunner. (8)
 8. The injection mold according to claim 7, wherein said hotrunner has a valve gate structure. (9)
 9. The injection mold accordingto claim 8, wherein said gate is closed by means of said valve gatestructure after said cut punch has moved to said first position.
 10. Theinjection mold according to claim 1, wherein said injection mold is usedto mold the resin molded product having an opening and has said resinreservoir and said cut punch inserted into said resin reservoir, saidresin reservoir and said cut punch being provided so as to correspond tothe opening of the resin molded product. (10)
 11. The injection moldaccording to claim 1, wherein a depth as viewed in a moving direction ofsaid cut punch in said resin reservoir is 1.5 to 10 times an openingdistance of said communicating portion. (11)
 12. The injection moldaccording to claim 1, further comprising a pushing device for removingthe resin solidified portion which is attached to said distal end ofsaid cut punch, said pushing device being slidably mounted inside saidcut punch, so as to be independent of said cut punch. (17)
 13. Aninjection mold comprising: a fixed die and a movable die which form acavity into which a molten resin material is injected via both a runnerand a gate provided in the fixed die, the gate being connected to thecavity through a resin reservoir formed by recessing the fixed dietoward the gate; and a cut punch provided on the side of the movable diethat confronts the gate through the resin reservoir, the cut punch beingmovable so that the cut punch can be inserted into the resin reservoirso as to be in slidable contact with the resin reservoir, wherein whenthe molten resin material is being charged into the cavity, a distal endof the cut punch which extends in a moving direction of the cut punchthat is toward the resin reservoir is located between the resinreservoir and the cavity at such a position as to open a communicatingportion that allows the resin reservoir and the cavity to communicatewith each other so that the molten resin material is introduced into thecavity via the resin reservoir; wherein when an inner portion of theresin material that is present in the resin reservoir is still moltenand a portion of the resin material thereof that is in direct contactwith the cooled dies is gradually solidified after the molten resinmaterial has been charged into the cavity and the resin reservoir, thecut punch moves toward the gate so that the cut punch is inserted intothe resin reservoir, whereby the cut punch not only closes thecommunicating portion while forcibly pushing the still molten resinmaterial present in the resin reservoir back into the gate, but alsocuts the resin material at the communicating portion so that a resinmolded product formed in the cavity is separated from a resin solidifiedportion formed in the resin reservoir, wherein the runner contacts theresin reservoir, and the gate is disposed at the point where the runnercontacts the resin reservoir, such that the gate does not protrude intothe resin reservoir, and wherein an undercut portion is provided at aperiphery of the distal end of the cut punch, disposed closer to an edgeportion of the distal end than a center portion of the distal end, theundercut portion serving to hold the resin solidified portion formed inthe resin reservoir at the time of mold opening.(6)
 14. An injectionmold comprising: a fixed die and a movable die which form a cavity intowhich a molten resin material is injected via both a runner and a gateprovided in the fixed die, the gate being connected to the cavitythrough a resin reservoir formed by recessing the fixed die toward thegate; and a cut punch provided on the side of the movable die thatconfronts the gate through the resin reservoir, the cut punch beingmovable so that the cut punch can be inserted into the resin reservoirso as to be in slidable contact with the resin reservoir, wherein whenthe molten resin material is being charged into the cavity, a distal endof the cut punch which extends in a moving direction of the cut punchthat is toward the resin reservoir is located between the resinreservoir and the cavity at such a position as to open a communicatingportion that allows the resin reservoir and the cavity to communicatewith each other so that the molten resin material is introduced into thecavity via the resin reservoir; wherein when an inner portion of theresin material that is present in the resin reservoir is still moltenand a portion of the resin material thereof that is in direct contactwith the cooled dies is gradually solidified after the molten resinmaterial has been charged into the cavity and the resin reservoir, thecut punch moves toward the gate so that the cut punch is inserted intothe resin reservoir, whereby the cut punch not only closes thecommunicating portion while forcibly pushing the still molten resinmaterial present in the resin reservoir back into the gate, but alsocuts the resin material at the communicating portion so that a resinmolded product formed in the cavity is separated from a resin solidifiedportion formed in the resin reservoir, wherein the runner contacts theresin reservoir, and the gate is disposed at the point where the runnercontacts the resin reservoir, such that the gate does not protrude intothe resin reservoir, and wherein a depth as viewed in a moving directionof the cut punch in the resin reservoir is 1.5 to 10 times an openingdistance of the communicating portion. (11)
 15. An injection moldcomprising: a fixed die and a movable die which form a cavity into whicha molten resin material is injected via both a runner and a gateprovided in the fixed die, the gate being connected to the cavitythrough a resin reservoir formed by recessing the fixed die toward thegate; a cut punch provided on the side of the movable die that confrontsthe gate through the resin reservoir, the cut punch being movable sothat the cut punch is operative to be inserted into the resin reservoirso as to be in slidable contact with the resin reservoir, and acommunicating portion, formed between the resin reservoir and the cavitywhen the cut punch is not in slidable contact with the resin reservoir,the communication portion allowing the resin reservoir and the cavity tocommunicate with each other so that the molten resin material isintroduced into the cavity via the resin reservoir, wherein a moldedresin product is formed when the cut punch device moves through theresin reservoir towards the gate to close the communication portion,which cuts the resin material at the communication portion so that theresin molded product formed in the cavity is separated from a resinsolidified portion formed in the resin reservoir, wherein the cut punchmoves at a time when an inner portion of the resin material that ispresent in the resin reservoir is still molten and a portion of theresin material thereof that is in direct contact with cooled dies isgradually solidified after the molten resin material has been chargedinto the cavity and the resin reservoir and wherein an undercut portionis provided at a periphery of the distal end of the cut punch, disposedcloser to an edge portion of the distal end than a center portion of thedistal end, the undercut portion serving to hold the resin solidifiedportion formed in the resin reservoir at the time of mold opening. (15)16. The injection mold according to claim 15, further comprising apushing device for removing the resin solidified portion which isattached to the distal ends of the cut punch, the pushing device beingslidably mounted inside the cut punch, so as to be independent of thecut punch. (17)
 17. An injection mold comprising: a fixed die and amovable die which form a cavity into which a molten resin material isinjected via both a runner and a gate provided in the fixed die, thegate being connected to the cavity through a resin reservoir formed byrecessing the fixed die toward the gate; and a cut punch provided on theside of the movable die that confronts the gate through the resinreservoir, the cut punch being movable so that the cut punch can beinserted into the resin reservoir so as to be in slidable contact withthe resin reservoir and having a distal end which extends in a movingdirection of the cut punch that is toward the resin reservoir, thedistal end located between the resin reservoir and the cavity at such aposition as to open a communicating portion that allows the resinreservoir and the cavity to communicate with each other so that themolten resin material is introduced into the cavity via the resinreservoir; and means for driving the cut punch when an inner portion ofthe resin material that is present in the resin reservoir is stillmolten and a portion of the resin material thereof that is in directcontact with the cooled dies is gradually solidified after the moltenresin material has been charged into the cavity and the resin reservoir,wherein the cut punch is driven toward the gate so that the cut punch isinserted into the resin reservoir, whereby the cut punch not only closesthe communicating portion while forcibly pushing the still molten resinmaterial present in the resin reservoir back into the gate, but alsocuts the resin material at the communicating portion so that a resinmolded product formed in the cavity is separated from a resin solidifiedportion formed in the resin reservoir, and wherein an undercut portionis provided at a periphery of the distal end of the cut punch, disposedcloser to an edge portion of the distal end than a center portion of thedistal end, the undercut portion serving to hold the resin solidifiedportion formed in the resin reservoir at the time of mold opening.(18)